1. Field of the Invention
The present invention relates to a method and a device for generating respective drive signals for four switch elements of a power output stage which are arranged in a bridge circuit, and relates as well as to a power output stage. In particular, the invention is provided for use in a power output stage which permits the generation of high output voltages and currents with high precision, such as an output stage of a gradient amplifier in a magnetic resonance tomography device, for example, or other fields, such as in devices for inductive heating.
1. Description of the Prior Art
A gradient amplifier in a magnetic resonance tomography device must provide voltages on the order of magnitude from several hundred volts to several thousand volts during a measurement procedure, in order to generate a precisely controlled current in a gradient coil. The current characteristic has maxima up to several hundred amps. The predetermined current flow must be precisely maintained to within a few mA.
To satisfy these high requirements, switched output stages are usually employed. German OS 40 24 160 discloses such an output stage of a gradient amplifier, wherein four switch elements are arranged in an H-bridge circuit. Pairs of these switch elements are connected in series and are connected to a supply voltage. A modulator generates a drive signal for each of the switch elements.
German OS 43 04 517 discloses a modulator for a gradient amplifier which generates respective drive signals for four switch elements that are arranged in a bridge circuit by comparing a manipulated variable to a triangular voltage.
In terms of circuitry, modulators such as described in German OS 40 24 160 and German OS 43 04 517 have a high outlay. An analog circuit which generates the required drive signals requires elements with tight tolerances and a large printed circuit board area. Moreover, an expensive calibration is necessary in connection with production and maintenance (servicing). This is true particularly with respect to the generation and maintenance of dead times in the alternating of switches within a bridge circuit.